The massive depletion of CD4+ T cells within the gut associated lymphoid tissues (GALT) during acute HIV/SIV followed by sustained CD4+ T cell loss during chronic infection leads to progressive damage to the GALT and the overlying mucosal epithelium. These events contribute to immune dysfunction, viral loads and the rate of disease progression over time. The CD4+ T cells traffic to the GALT by signals generated within the GALT which include the release of retinoic acid which in turn leads to upregulation of the ?4?7 integrin and CCR9 on the cell surface. Cells expressing ?4?7 and CCR9 selectively home to the GALT by interacting with their cognate receptors MAdCAM and CCL25 expressed by epithelial cells lining the intestinal wall. Recent data show that ?4?7, besides serving as the homing receptor, also serves as an alternate receptor for many strains of HIV and SIV whose V1/V2 env segments have recognition 'motifs' for ?4?7 similar to MAdCAM its natural ligand. In addition, sequences localized to the V1/V2 and V3/C4 env region contain residues which, if deglycosylated, markedly enhance the affinity of the virus to bind ?4?7. Such ?4?7 high affinity binding HIV-1 is thought to be preferentially transmitted via the mucosal route. Our lab has recently shown that the administration of a primatized anti??4?7 mAb, just prior to and during acute IV and intra-rectal SIV infection, leads to markedly reduced viral loads in the GALT and provides, for the first time, tools to examine in detail the events that occur during acute infection. The studies proposed herein are aimed at: 1) determining the clinical utility of these previous findings by determining if ?4?7 administration is effective in lowering GALT viral loads in animals a) infected intravaginally, b) post SIV infection, c) infected with low repeated doses intravaginally to mimic natural transmission, and d) if a small molecule inhibitor of CCR9 alone or with ?4?7 mAb administration leads to more effective and prolonged control of GALT viremia; 2) defining the histopathological analysis of the gut tissues with a focus on delineating the role of proteins involved in maintaining gut tissue integrity, defining the kinetics of bacteril translocation and the physiologic measurement of gut tissue permeability; and 3) determining the mechanisms involved by which ?4?7 induces its effect a) by determining whether ?4?7 mAb mediates its effect via blocking the receptor function of the ?4?7 or by blocking the trafficking of ?4?7 expressing cells by using recombinant replication competent SIV env constructs that do or do not bind ?4?7, b) by determining whether the in vivo effect of anti-??4?7 treatment is influenced by the level of env glycosylation using W.T. & recombinant env deglycosylated SHIV's that bind ?4?7 with low v/s high affinity, c) determine the tissue/organ localization of virus and CD4+ cells in the control and ?4?7 administered animals using a newly optimized real time LIVE PET-CT scanning technique developed by our lab. The realization that effective HIV vaccines require to elicit not only broad neutralizing Abs and effective virus specific CTL's but also means to prevent GALT injury which is intimately associated with disease progression, highlight the importance of these studies.